Soft tissues such as tendons and ligaments are generally attached to bones by small collagenous fibers. These fibers are strong, but permit tendons and ligaments to be flexible. When soft tissue is torn away from the bone, a surgeon is often required to reattach the soft tissue, or a replacement tissue graft to the bone with one of more surgical anchors or other surgical fixation implants. Surgical fixation implants, such as screws, surgical anchors, graft anchors, and surgical pins, may be implanted into a patient's bone to reattach or fix soft tissue or to reinforce damaged bone. Fixation implants are implanted into bone using extensive surgical procedures, and more recently, using arthroscopic surgical techniques. A growing recent trend is to manufacture fixation implants from bio-inert and bio-absorbable materials so that native bone tissue may gradually absorb the fixation implant and grow into the space occupied by the fixation implant to replace the implant with native bone tissue.
A frequently performed procedure where fixation implants are routinely used to attach a ligament graft to a bone is anterior cruciate ligament (ACL) reconstruction. This procedure generally involves removing the torn or damaged ACL and forming tunnels in the distal femur and proximal tibia in close proximity to the original ACL attachments sites. A replacement graft may be harvested from the patellar tendon (along with a portion of the patella and the tibia, i.e., a bone-to-bone graft), from the hamstring tendons of the patient, or from another donor. The graft may be pulled through or into the bone tunnels and fixed in the bone tunnels with a graft anchor. The graft may then be left to function as a new ACL. The tibial portal is more commonly used for this procedure, but the antero-medial portal has seen some increased use recently.
Rigid fixation of the graft anchor is recognized as an important factor for the long-term success of ACL reconstruction procedures. Rigid fixation is difficult to initially obtain, and even more difficult to maintain throughout the life of the graft anchor.
One type of graft anchor commonly used to fix the ligament graft in ACL reconstructions is an interference screw, which biases a graft segment, against a wall of a bone tunnel (e.g., formed in the femur). However, use of interference screws may result in damage to the graft. For example, the threads of the interference screw, and the bone tunnel wall may cut or abrade the graft as the interference screw is advanced into the bone tunnel. Further, advancing the interference screw into the bone tunnel over the graft may twist the graft in a way that proper graft tension is lost or exceeded. In more severe cases, over-tightening of the interference screw may cause catastrophic failure of the bone tunnel known as tunnel blowout.
Tunnel sealing is another important consideration in ACL reconstructions, as it is known that synovial fluid acting on the interference screw may cause bone tunnel enlargement. For example, improper tunnel sealing may result in graft loosening, such as a bungee effect or windshield wiper effect.
Attempts have been made to overcome the above deficiencies in the prior art. However, such attempts involve multi-piece fixation implants which require forming additional holes or tunnels into the bone (e.g., transversely to the initial attachment tunnel), and thus result in increased procedure duration and complexity, implant costs, surgeon errors and failure rates, and prolong patient recovery time.